1. Field of the Invention
The invention relates generally to volume holographic data storage and more particularly to volume holographic data storage using doped high optical quality glass.
2. Description of the Related Art
In holography, a photosensitive medium is exposed to an interference pattern that is generated by intersecting a spatially-modulated information-carrying object beam with a coherent reference beam. The photosensitive medium records the interference pattern through some spatially-modulated change in the optical properties of the medium. The recorded information can then later be retrieved by exposing the medium to the reference beam, which is diffracted by the medium in such a manner as to reconstruct the object beam.
In volume holographic data storage, the object beam encodes a page of input data and the interference pattern is recorded three-dimensionally in the photosensitive medium. If the photosensitive medium is thick enough, multiple pages of input data can be recorded or multiplexed in the same volume element, for example by varying the angle between the object and reference beams or by varying the wavelength of the object and reference beam.
Although volume holographic data storage has been known for several decades and volume holograms have been recorded in a variety of media, commercial development of volume holographic data storage has been hampered by lack of a storage medium that meets all the requirements for technical and commercial feasibility. In particular, the characteristics of the ideal optical storage medium include high optical quality and low scatter, a large refractive index modulation, high sensitivity, long shelf life and low material costs. Further, an ideal material should not require additional chemical or physical treatment to fix it for long term storage. The material should be erasable, yet should be able to be read without accidentally erasing. For a description of desirable criteria for a volume holographic storage medium, see, for example, G. T. Sincerbox, "Holographic storage--the quest for the ideal material continues," Opt. Mat. 4, 370-375 (1995) and M. P. Bernal, G. W. Burr, H. Coufal, R. K. Grygier, J. A. Hoffnagle, C. M. Jefferson, R. M. Macfarlane, R. M. Shelby, G. T. Sincerbox and G. Wittman, "Holographic-Data-Storage Materials," MRS Bulletin, 51-60 (September, 1996), both incorporated herein by reference. Even. the very best material previously identified for holographic storage applications, lithium niobate, suffers from a number of deficiencies, including relatively low sensitivity, erasure of data upon readout and insufficient optical quality.
A volume holographic storage medium of hydrogen-treated germano-silicate glass is disclosed in Partovi, A. et al "Volume Holographic Storage in Hydrogen Treated Germano-Silicate Glass" Appl. Phys. Lett. 64(4) (1994), pp 821-823. Since hydrogen loading is required for the material to function as a recording medium, its sensitivity diminishes over time as hydrogen diffuses out of the medium.
U.S. Pat. No. 5,383,038 to Lawandy describes a volume holographic medium made up of bulk glass containing embedded semiconductor microcrystallites. The storage medium is described in connection with a process of operating a holographic memory in which the glass is irradiated with two light beams; with one beam having a wavelength that is half the wavelength of the other beam. The information is read out at only the shorter wavelength. This method has the disadvantage that the particular high energy wavelength used in recording the hologram must be used in reading the hologram.